If you're using OpenDNS, you can try to access Pinterest and then when it doesn't work, take a look at the recently blocked domains and it should give you some clue as to which CDNs or other domains Pinterest is attempting to use.

I've got much the same problem in reverse trying to help a local school admin block YouTube on their network. I really think these "Social" Networks pull this kind of shit intentionally just to make filtering it more (and completely unnecessarily) difficult - Can U say 'Scattered to the four winds"?: